It is usually a good idea to only have one Mailchimp Audience.
Think of this Audience as your central email database.
You can have all of your (subscribed) contacts there.
Some organisations that I train send their Mailchimp campaigns to everyone within the Audience.
Others use Audience fields, Tags, Groups and Segments in order to send specific messages.
For example, you could use Groups for countries and / or counties.
You could use Tags for different products or services which you supply.
I have not found an organisation yet that couldn’t make use of this functionality.
If you send a tailored message to someone that only contains information that is relevant to them, you will get better results.
PS Here is my Mailchimp training website.